1. Field of the Invention
The invention generally relates to current control circuits, and more particularly to a system for maintaining low magnitude currents, such as beam currents in light valves at a substantially constant value in high voltage applications.
2. Description of the Prior Art
Projection television displays have utilized light valves which are suitable for projecting images onto remote display surfaces. Such light valves have been incorporated in professional large screen color and monochrome television projectors of the type sold by the General Electric Company and described, for example, in General Electric Company Publication No. VDEO-077.
Light valves have been described in the literature, including the following publications: "Projection Color Television Display Using a Single Beam Oil Film Light Valve," Dr. W. E. Good et al., April 1981, TIS Report No. 81VDE-001, General Electric Company; "Recent Advances In the Single-Gun Television Color Light-Valve Projector," W. E. Good, Simulators & Simulation, Design, Applications, and Techniques, SPIE Vol. 59, 1975; and "A New Approach To Color Television Display and Color Selection Using a Sealed Light Valve," W. E. Good, Proceedings Of the National Electronics Conference, Vol. XXIV, 1968.
While the foregoing literature describes the construction and operation of light valves, it is important to point out for purposes of the subject invention that the intensity of the electron beam or beam current within the light valve must be maintained at a virtually constant value in order to avoid variations in the intensity of light emitted by the light valve. Any undesirable variations in the intensity of the electron beam interferes and masks the desired modulation conditions and, as suggested, such undesired variations are detrimental to the proper operation of the light valve since undesired optical intensity variations are introduced into the displayed image.
Systems for maintaining a constant intensity of the light valve electron beam have been proposed. Systems of this type are described in U.S. Pat. Nos. 3,325,592 to W. E. Good et al., issued June 13, 1967, and 3,619,717 to Lee et al., issued Nov. 9, 1971. Both of the foregoing patents have been assigned to the Assignee of the subject invention.
The Lee et al patent discloses an electron beam current regulator for a light valve which utilizes feed-back to sense the light valve beam current to control the frequency of a free-running relaxation oscillator which includes a gas-filled glow discharge tube. The glow discharge tube is utilized to discharge the charge stored on a timing capacitor. The resulting pulses at the output of the relaxation oscillator are converted to a voltage by a frequency-to-amplitude converter, the output of which is compared to a reference voltage. The output of the comparator is used to regulate the voltage at the control grid of the light valve. The Lee et al circuit has a number of disadvantages. Primarily, it relies on a neon glow discharge tube. The precise electrical characteristics of such tubes are difficult to control and their characteristics vary over the age of the tube. Since the tube characteristics are crucial in establishing the frequency of the relaxation oscillator, and regulation of the beam current relies on the frequency of oscillation, the Lee et al circuit is not sufficiently accurate and reliable. Although initial accuracy can be maximized by careful hand selection of the neon tubes, the operation of the circuit deteriorates with aging of the tube. An additional disadvantage of the Lee et al arrangement is that the use of a neon glow discharge tube limits the minimum width of the pulses because of the firing time of the neon tube. The wide pulses introduce error into the system because the time averages of the pulses are measured.
Starting in the early 1970's, the General Electric Company has used beam current regulating circuitry in its PJ5000 series video projectors which is a modification of the circuit disclosed in the Lee et al patent. The circuitry in the PJ5000 series video projectors also makes use of a neon tube. However, instead of being used to discharge a timing capacitor as in Lee et al, the neon tube in the PJ5000 circuitry charges a capacitor circuit which then provides the current to the output. Again, the inability to control the operating parameters of the neon tube and keeping those parameters constant has resulted in a wide fluctuation in tolerances particularly over the life of the neon tube. The disadvantages in using a neon tube discussed above in connection with the Lee et al patent are equally applicable to the PJ5000 circuitry.
Additionally, light valves, like numerous other active current control devices, require high voltages on the order of kilovolts. This has presented a problem in the design of current regulating circuitry which must sense and measure extremely low currents, on the order of micro-amps, at high impedance, by using low voltage control circuitry which must be adequately isolated from the sources of high voltage.